Vol. 70, No. 2 (2021)

I L-shell X-rays from near Bohr-velocity I20+ ions impacting on various targets
Zhou Xian-Ming, Wei Jing, Cheng Rui, Zhao Yong-Tao, Zeng Li-Xia, Mei Ce-Xiang, Liang Chang-Hui, Li Yao-Zong, Zhang Xiao-An, Xiao Guo-Qing
2021, 70 (2): 023201. doi: 10.7498/aps.70.20201236
Abstract +
The L-shell X-ray emissions of iodine are investigated as a function of target atomic number for 4.5-MeV I20+ ions impacting on Fe, Co, Ni, Cu and Zn targets. Six distinct L-subshell X-rays are observed. The energy of the x-ray has a blue shift compared with the atomic data. The relative intensity ratio of Lβ1, 3, 4 and Lβ2, 15 to Lα1, 2 almost increase linearly with the target atomic number increasing. The ratio of I(Lι) to I(Lα1, 2) and I (Lγ2, 3, 4, 4') to I(Lγ1) are approximately proportional to the square of target atomic number. It is indicated that during the interaction of highly charged heavy ions with atom in the energy region near the Bohr velocity, the inner-shell process is mainly caused by the close-range collisions below the surface. There, the projectile not only has enough time to capture electrons from the target atom to be neutralized, but also has enough kinetic energy to ionize the inner-shell electron by coulomb interaction. At the balance between electron capture and ionization, the outer-shells of M, N, O etc. could be multiply ionized. The extent of multiple ionization increases with the target atomic number increasing. That leads to the energy shift, resulting in the change of the relative intensity ratio for the L-subshell X-ray. The smaller the atomic fluorescence, the larger the enhanced fluorescence caused by multiple ionization.
Applications of representation transformation and secular perturbation theory to coupled Duffing equations
Li Chao-Gang, Wang Mao-Sheng, Fang Quan, Peng Xue-Cheng, Huang Wan-Xia
2021, 70 (2): 024601. doi: 10.7498/aps.70.20201057
Abstract +
In physics, the non-linear mode coupling is an important strategy to manipulate the mechanical properties of a vibrational system. Compared with the single-mode nonlinear system, the complex systems with two- or multi-mode nonlinear coupling have garnered considerable attention, among which the analytical solutions to the coupled Duffing equations are widely studied to solve nonlinear coupling. The fact is that the solving of the Duffing coupling equations generally starts with the eigenmodes solution of the linear equations. The trial solution of the coupled equations is the linear superposition of the eigenmodes. Under the secular perturbation theory and similar conditions, the Duffing coupling equation degenerates into two decoupled equations. However, thus far most of the solution methodologies are too complicated to unravel the underlying physical essence clearly. In this paper, first, by applying the representational transformation to the linear terms of the first-order coupled Duffing equations and the secular perturbation theory for the nonlinear terms, a decoupled expression of the first-order Duffing equations is derived, which can be solved more straightforwardly. Subsequently, in order to verify the correctness of the method, we design a coupled tuning fork mechanical vibration system, which consists of two experimental instruments to provide driving force and receive signals, two tuning forks and springs. The amplitude spectra are measured by an experimental instrument of forced vibration and resonance (HZDH4615), which provides a periodic driving signal for the tuning fork. The numerical fitting by software is employed to clarify the mechanism of the spectrum. Theoretically, the obtained fitting parameters can also evaluate some important attributes of the system. Most strikingly, due to the nonlinear coupling the splitting of the resonant peak and the phenomenon of “hysteresis loop” are clearly observed in the experiment. The research shows that the experimental results perfectly match the theoretical results obtained before. The method of solving coupled nonlinear equations in this article provides a solution and improvement of flexible adoption of nonlinear theory. On the other hand, it can be extended to coupled light and electricity systems, offer certain guidance for understanding the dynamic behavior of coupled systems, and will be conductive to the quantitative examination of numerous nonlinear coupling devices.
Initiation of nanosecond-pulsed discharge in water: Electrostriction effect
Li Yuan, Li Lin-Bo, Wen Jia-Ye, Ni Zheng-Quan, Zhang Guan-Jun
2021, 70 (2): 024701. doi: 10.7498/aps.70.20201048
Abstract +
Underwater nanosecond-pulsed discharges have been widely utilized in numerous industrial applications. The initial stage of nanosecond-pulsed discharge in water contains extremely abundant physical processes, however, it is still difficult to reveal the details of charge transportation and multiplicative process in liquid within several nanoseconds by currently existing experimental diagnostic techniques. Up to now, the initiation mechanism of underwater nanosecond discharge has been still a puzzle. In this paper, we develop a two-dimensional axially symmetric underwater discharge model of pin-to-plane, and numerically investigate the electrostriction process, cavitation process, and ionization process in water, induced by nanosecond-pulsed voltage. The negative pressure in water caused by tensile ponderomotive force is calculated. The creation of nanoscale cavities (so-called nanopores) in liquid due to negative pressure is modeled by classical nucleation theory with modified nucleation energy barrier. When estimating the temporal development of nanopore radius, a varying hydrostatic pressure is considered to restrain the unlimited expansion of nanopores. We estimate the electron generation rate by the product of the generation rate of incident electrons and the number density of nanopores. The simulation results show that cavitation occurs in liquid within several microns from pin electrode due to the electrostriction, which results in the formation of a large number of nanopores. The expansion of nanopore, caused by electrostrictive pressure on nanopore surface, provides a sufficient acceleration distance for electrons. The impact ionization of water molecules can be triggered by energetic electrons, leading the local liquid to be ionized rapidly. The effects of nanopores on rapid electron generation in water are discussed. Once nanopores are formed, the electrons can be generated in the following ways: 1) Field ionization of water molecules on the nanopore wall continuously provides seed electrons; 2) the seed electrons accelerated in nanopores enter into the liquid and collide with water molecules, resulting in the rapid increase of electrons. It can be inferred that the randomly scattered nanopores act as micro-sources of charges that contribute to the continuing ionization of liquid water in cavitation region near pin electrode. Electrostriction mechanism provides a new perspective for understanding the initiation of nanosecond-pulsed discharge in water.
SPECIAL TOPIC—Construction of functional devices toward atomic-scale: Funda mentals and frontiers
Nucleic acid-metal complex and its application in atomic-scale manufacturing
Li Sheng-Kai, Hao Qing, Peng Tian-Huan, Chen Zhuo, Tan Wei-Hong
2021, 70 (2): 028102. doi: 10.7498/aps.70.20201430
Abstract +
Atomic-scale and close-to-atomic scale manufacturing, a frontier hot issue in international academic research, is a cutting-edge manufacturing technique in which atoms are directly used as the manipulation object and atomic-scale structures with specific functions are established to meet the requirements for mass productions. This review focuses on precise atomic-scale manufacturing technology of nucleic acid materials. Firstly, the basic structures and functions of nucleic acid materials are introduced, and the basic principles of the interaction between DNA and metal atoms are discussed. Then the development process and breakthrough progress of nucleic acid materials-mediated precise atomic-scale manufacturing are introduced from the aspects of natural nucleic acid materials, artificial base “molecular elements”, and nucleic acid nanostructures. Finally, the challenges and opportunities in this field are systematically summarized and some suggestions for future development are given.